The Golden Pumpkin is a special item obtained at the Spirit’s Eye Festival, its only purpose is selling it for a profit of 2,500g.
To receive the Golden Pumpkin, head to the event. Make your way through the maze until you reach a sign that has a question mark on it. Go through the bushes to the left of the sign and again move north into a cave. Continue east and then south until you reach the treasure chest with the Golden Pumpkin.